THE LAYERS OF BARK. 45 which they form is termed muriform parenchyma (Figs. 91, B, i, i, and 26). The medullary rays constitute the silver grain of cabinet-makers and carpenters. (6) The Bark or Cortex.-The bark (Fig. 85, c, c,) is organically connected with the wood by means of the medullary rays and cambium-layer. When the stem is first formed the bark is entirely composed, like the pith, of parenchyma, but, as soon as the wood begins to be developed on the outside of the pith, certain cells which lie nearer the surface of the stem make their appearance, which develop into liber-cells. Externally to these lie other parenchymatous cells, the inner ones of which form the green layer of the bark, c, while the outer cells become developed into the cork tissue, b, and these again are invested by colorless cells, forming the epidermis, a, so that the bark, when fully formed, consists of two distinct systems namely, an internal or fibro-vascular, and an external or parenchymatous. Further, the parenchymatous system, as just noticed, exhibits a separation into two portions; the fully developed bark accordingly presents three distinct layers. a. The Liber, Inner Bark, or Phloëm, composed of true bast tissue, consists of narrow elongated cells with thickened and flexible walls, mixed with parenchymatous tissue, and usually laticiferous vessels and sieve-tubes. b. The Cellular Envelope, Green Layer, or Phelloderm, the middle layer of the bark, consists essentially of thinsided, usually angular or prismatic, parenchymatous cells, which are loosely connected, and thus leave between their walls a number of interspaces. Its cells contain an abundance of chlorophyl, which gives the green color to young bark, and hence the name of green layer, by which it is also commonly distinguished. c. Corky Layer, or Periderm, the outer layer of the bark, consists of one or more layers of tabular cells, forming by their union a compact tissue, or one without inter 46 GROWTH OF BARK. spaces. It is this layer which gives to the young bark of trees and shrubs their peculiar hues, which are generally brownish or some color approaching to this; or rarely it possesses more vivid tints. In some plants, as in the Cork-oak, this layer becomes excessively developed and forms the substance called cork, and hence the name corky layer which is commonly applied to it. Growth of the Bark.-The bark, except the middle layer, develops in an opposite direction to that of the wood. The two outer layers generally cease growing after a few years, and become dead structures on the surface of the tree; but the inner bark continues to grow throughout the life of the individual, by the addition of a new layer annually on its inner surface from the cambium. These layers are commonly so thin when separated that they appear like the leaves of a book, and hence the supposed origin of the term liber applied to the inner bark. The outer layers of the bark, after a certain period, which varies in different plants, become cracked in various directions in consequence of the pressure which is exerted upon them by the growth of the wood and liber beneath, and thus assume a rugged appearance, as in the Elm and Cork-oak, and are commonly thrown off in large pieces, or in plates or layers of various sizes and appearances. The epidermis in all cases separates early and is replaced by cork-cells. The new layers of wood, the cambium-layer, and the recently formed liber, are the parts of an exogenous stem which are alone concerned in its active development and life. B. Endogenous or Monocotyledonous Stem.In this country we have no indigenous trees or large shrubs which exhibit this mode of growth, although we have numerous herbaceous plants, such as Grasses, Rushes, and Sedges, which are illustrations of endogenous structure. But it is in the warmer regions of the earth, and especially in the tropics, where we find the most striking MONOCOTYLEDONOUS STEMS. 47 and characteristic illustrations of such stems, and of all such the Palms are by far the most remarkable. When we make a transverse section of a Palm stem, it presents, as we have seen (page 37), no such separation of parts into pith, wood, medullary rays, and bark, as we have described as existing in an Exogenous stem; but the fibro-vascular system is seen to consist of bundles (Figs. 86, f, and 92, A, b, c, d), which have no tendency to collect together and form rings of wood as in Exogenous stems, but are arranged separately from one another in the mass of parenchymatous cells (Figs. 86, m, and 93, FIG. 92.-Diagram of a Monocotyledonous stem. A. Transverse section. B. Vertical section. a, a. Parenchymatous tissue. b, b. ¡Pitted vessels. c. Wood-cells. d, d. Spiral vessels. (After Carpenter.) A, a), of which the fundamental tissue is composed. The whole is covered externally by a fibrous and parenchymatous layer, which is called the false bark or rind (Fig. 86, b); because this is not a distinct and parallel formation to the wood, as is the case with the bark of Exogenous stems, but is formed essentially by the ends of the fibro 48 GROWTH OF FIBRO-VASCULAR BUNDLES. vascular bundles, as will be presently noticed, and can not therefore be separated from the mass beneath. In annual or herbaceous Endogenous stems the parenchyma between the fibro-vascular bundles is soft and delicate, but in trees which grow to any height, as Palms, the cell-walls become thickened and hardened, and thus form the tissue termed sclerenchyma, which ultimately binds the bundles into a solid hardened mass resembling wood. d In Origin and Growth of the Fibro-Vascular Bundles. These bundles have their origin in the punctum vegetationis of the stem, and are fully developed with its growth upward and outward into the leaves, and downward and outward toward the circumference of the stem. other words, the bundles may be traced to the leaves, from which organs they are at first directed toward the interior of the stem (Fig. 93, a, b, c, d), along which they descend generally for some distance, and then gradually curve outward again and terminate at the circumference. Hence, when we make a vertical section of an Endogenous stem, these bundles are seen to intersect one another in various ways. FIG. 93.-Diagram showing the course of the fibro-vascular bundles of a dles. The fibrovascular bundles in their course down the stem also become altered Monocotyledonous in their structure. When they first origistem. a, b, c, d. Fibro-vascular bun- nate they consist of spiral, pitted, and other vessels, mixed with parenchymatous and woody tissues (Fig. 93, B, b, b, c, d, d). In their descent they gradually lose their spiral and other vessels, so that when they terminate at the circumference they consist chiefly of liber-cells bound together by parenchyma. The rind or false bark (Fig. 86, b) of Endogenous stems is thus chiefly formed of the ends of the fibro-vascular bundles, and hence we see the principal reason why this rind can FIBRO-VASCULAR BUNDLES. 49 not be separated, as the bark of Exogenous stems, from the tissues beneath. As the bundles are only endogenous for a portion of their course, the term has been generally discarded of late years by botanists, who use instead that of monocotyledonous, as the embryo of such plants has but one cotyledon. As the fibro-vascular bundles of an endogenous stem, in the course of their successive development, are always directed at first toward the center, it must necessarily follow that those previously formed will be gradually pushed outward, for which reason the outer part of a transverse section will always exhibit a closer aggregation of bundles than the inside (Figs. 86, f, and 91, A, b, c, d). In such stems, therefore, the hardest part is on the outside, and the softest inside, directly the reverse of what occurs in those of exogenous growth. The lower portion of such stems also, in consequence of the descent of the fibro-vascular bundles, the constituents of which become, moreover, more or less thickened in their interior, will be harder than the upper. The rind in like manner, at the lower part, will become harder, from the greater number of liber-cells which terminate in it. As endogenous stems increase in diameter, partly by the deposit of fibro-vascular bundles in their interior, and partly by the general development of the parenchymatous tissue in which they are placed, it follows that, as soon as the rind has become thus hardened by the liber-cells and other causes, it is not capable of further distension, and the stem will consequently become at length choked up by the bundles which continue to descend, and further growth is then impossible. It is evident, therefore, that endogenous stems, unlike those of exogenous growth, can not increase in diameter beyond a certain limit, and that from the same causes also they can not live beyond a certain age. Growth by Terminal Buds.—In Palms, and most |